The present invention relates to a filter device and a branching filter using the filter device, in particular to a technique effectively applicable to an improvement of sharpness of filter characteristic between pass band and prohibition band in the filter device.
In recent years, a mobile communication terminal equipment, such as a portable telephone and the like has been rapidly developed. Subsequently, it is desired that the mobile communication terminal equipment is small in size, light in weight and capable of saving electric power for the purpose of convenience for portability thereof. In order that the mobile communication terminal equipment may be small in size, light in weight and capable of saving electric power, it is necessary that the filter device used therein is not only small in size and light in weight but also low in insertion loss. Further, from the view point of high speed and large capacity communication, it is also required that the filter device has a wide pass band characteristic.
In order to meet the requirement, many filter devices each of which a plurality of resonators are connected to each other like a comb, namely ladder type filters are used as a filter device of the mobile communication terminal equipment. An SAW resonator using surface acoustic wave (SAW), a piezo-electric resonator composed of a crystal vibrator using bulk acoustic wave (BAW), and the like is used as each of the resonators for constituting the ladder type filter. It has recently been known that the ladder type filter may be constituted by thin film resonators each using a thin film of piezo-electricity.
In order to realize a ladder type filter capable of having a wide pass-band characteristic, for example, Japanese unexamined patent publication Hei 10-93375, namely No. 93375/1998 discloses a technique that an inductance element is added in a parallel arm or that frequency differences are made between resonance frequencies of resonators formed in a serial arm and anti-resonance frequencies of resonators formed in a parallel arm.
It is, of course, necessary in a filter device that an insertion loss at a pass-band frequency fulfills a predetermined specification. In addition, it is also necessary in the filter device that a large amount of attenuation fulfilling a predetermined specification is obtained at a cut-off frequency (a pass-band frequency at the side of a communication partner in a transmission/reception filter device).
In addition, the filter device has not only pressure temperature dependence that temperature of the filter device varies characteristics thereof but also individual difference due to unevenness of manufacturing the filter device. It is therefore necessary not only that pass-band width capable of obtaining the insertion loss fulfilling the above specification is enlarged but also that cut-off band width capable of obtaining the amount of attenuation fulfilling the described specification is enlarged. As a result, the pass-band and the cut-off band become close to each other.
However, in the above-described conventional technique, especially when the pass-band and the cut-off band become close to each other, it is not possible to obtain sufficiently sharp attenuation characteristics. Consequently, when characteristics of the conventional filter device are varied due to environmental temperature for using the filter device and individual differences of the products thereof, desirable characteristics thereof cannot be obtained.